Marking easy, as this is probably a straightforward bug to fix.
Currently PDFHandler runs gs and convert in a pipeline (See PdfHandler::doTransform). It sticks 2>&1 on the end of the command to catch error output, however that doesn't apply to the whole pipeline, only the last command. Hence ghostscript (gs) errors are missed.
Recommended fix: remove the 2>&1, and instead of calling wfShellExec, call wfShellExecWithStderr. (This will also catch errors for other commands, like the resource limit things, so its better than using 2>&1 anyways).